Communication between cells in the nervous system underlies all complex behaviors, and occurs at specialized regions of the nerve cell called synapses. Synapses work by releasing chemical transmitter from a region called the active zone, which activates a neighboring cell. We propose to characterize the relationship between active zone function and structural organization within frog and mouse neuromuscular synapses. We hypothesize that neuromuscular active zones are assembled from a basic transmitter release building block: the unreliable single-vesicle release site consisting of a docked synaptic vesicle and its associated Ca2+ channels. We further hypothesize that major aspects of synaptic function and presynaptic homeostatic plasticity can be explained by changes in the number and organization of these single-vesicle release sites within active zones. Our approach is characterized by a seamless collaboration between three labs with expertise in computer simulations of cellular physiology (Dittrich lab), synaptic anatomy, physiology, and Ca2+ imaging (Meriney lab), and super-resolution imaging of the number and spatial distribution of synaptic proteins (Blanpied lab). Importantly, as part of this proposal, trainees from all three laboratories will receive crosstraining in each lab. We will use this collaborative approach to develop a comprehensive MCell computer model of the presynaptic transmitter release site that will significantly increase our understanding of the relationship between active zone organization and synaptic function. This insight will not only lead to a better understanding of presynaptic mechanisms of homeostatic plasticity but also aid in our understanding of synaptic diseases, which are known to underlie a large number of neurological disorders. Intellectual Merit: A significant number of neurological diseases are known to affect the synapse by targeting synaptic organization and function. While most research on this important topic has to date focused on postsynaptic adaptations, it has become increasingly clear that presynaptic homeostatic changes are likely to be just as important. Thus, a better understanding of the role of presynaptic structure and organization in synaptic function under both control and disease conditions is needed. Broader Impacts: The MCell model that we will develop will enhance our teaching mission in many ways. It will provide an example of unprecedented scale and realism for the illustration of nerve terminal structure and function. This material will be used in courses and programs at the University of Pittsburgh, the University of Maryland, and Carnegie Mellon University. These include undergraduate and graduate Neuroscience courses, a Computational Biology PhD program that spans PITT and Carnegie Mellon University, summer workshops, and web-based tutorials (www.mcell.org). These simulations will expand previous models that already have been converted into instructive 3D movies, which are routinely shown to a broad range of audiences during open houses, student visits or classroom teaching. This work will also provide source material for teaching examples tailored to high school outreach programs at the Pittsburgh Supercomputing Center, particularly the CMIST program (Computational Modules in Science Teaching, www.cmist.org) of the National Resource for Biomedical Supercomputing (NRBSC) directed by Dr. Dittrich. Our proposed work will have a broad impact on K-12 education, undergraduate teaching and training, graduate and post-graduate training, community outreach, STEM teaching, training at underrepresented minority institutions, and knowledge of synaptic function in the field. Dr. Meriney is a member of the Neuroscience outreach committee at the University of Pittsburgh (PITT), which organizes a variety of community events. Dr. Meriney's laboratory is in the Arts and Sciences College, so the proposed research would contribute to undergraduate teaching via undergraduate research participation in the proposed work, and changes to content for undergraduate courses based on new research insights. Dr. Dittrich will also train undergraduate students in his laboratory as participants in the proposed work. He is training faculty in the NSF funded TECBio REU program at the PITT and typically mentors 1-2 students in computational projects as part of the program. In addition, Dr. Dittrich is a training faculty in the PA Governors School for the Sciences, an intense summer program for talented high school students in Pennsylvania. Drs. Dittrich, Meriney, and Blanpied will bring graduate researchers and postdoctoral fellows into their labs who will directly participate in the proposed experiments, receive cross training in all three laboratories, and receive career training. Lastly, Dr. Ulises Ricoy (an under-represented minority faculty member) from Northern New Mexico College will visit during each summer to learn new research, teaching, and training tools to bring back to underrepresented minority undergraduates at Northern New Mexico College. This will expose these underrepresented minority students to an intense academic research environment and aid in their training and career planning.